Phenotypic characterization of Vibrio vulnificus biotype 2, a lipopolysaccharide-based homogeneous O serogroup within Vibrio vulnificus

A new understanding of Acanthamoeba castellanii: dispelling the role of bacterial pore-forming toxins in cyst formation and amoebicidal actions

Pore-forming toxins (PFTs) are recognized as major virulence factors produced by both Gram-positive and Gram-negative bacteria. While the effects of PFTs have been extensively investigated using mammalian cells as a model system, their interactions with the environmental host, Acanthamoeba castellanii remains less understood. This study employed high-throughput image screening (HTI), advanced microscopy, western blot analysis, and cytotoxicity assays to evaluate the impact of PFT-producing bacterial species on their virulence against A. castellanii. Our unbiased HTI data analysis reveals that the cyst induction of A. castellanii in response to various bacterial species does not correlate with the presence of PFT-producing bacteria. Moreover, A. castellanii demonstrates resistance to PFT-mediated cytotoxicity, in contrast to mammalian macrophages. Notably, Vibrio anguillarum and Ralstonia eutropha triggered a high frequency of cyst formation and cytotoxicity in infected A. castellanii. In summary, our findings reveal that A. castellanii exhibits a unique resistance to PFTs, unlike mammalian cells, suggesting its potential ecological role as a reservoir for diverse pathogenic species and its influence on their persistence and proliferation in the environment.

Genomic Insight into Zoonotic and Environmental Vibrio vulnificus: Strains with T3SS2 as a Novel Threat to Public Health

Vibrio vulnificus is a significant opportunistic pathogen with the highest fatality rate among foodborne microbes. However, due to a lack of comprehensive surveillance, the characteristics of isolates in China remain poorly understood. This study analyzed 60 strains of V. vulnificus isolated from diverse sources in Shanghai, including shellfish, crabs, shrimps, throat swabs of migratory birds, as well as seafood farming water and seawater. Identification of the genotypes was performed using PCR, and cytotoxicity was determined using an LDH assay. DNA was sequenced using Illumina NovaSeq followed by a bioinformatic analysis. The results demonstrated that a majority of the strains belonged to the 16S rRNA B-vcgC genotype. All strains carried five antibiotic resistance genes (ARGs), with some strains carrying over ten ARGs, mediating resistance to multiple antibiotics. Five strains possessed a highly abundant effector delivery system, which further investigations revealed to be a type III secretion system II (T3SS2), marking the first description of T3SS2 in V. vulnificus. Phylogenetic analysis indicated that it belonged to a different genetic lineage from T3SS2α and T3SS2β of V. parahaemolyticus. Bacteria with T3SS2 sequences were concentrated in coastal areas and mostly within the genus Vibrio in the global prevalence survey. Our study provides essential baseline information for non-clinical V. vulnificus and discovers the existence of T3SS2 in several strains which may be more virulent, thereby posing a new threat to human health.

Antibiotic resistance and virulence characteristics of Vibrio vulnificus isolated from Ningbo, China

Background

Vibrio vulnificus (V. vulnificus) is a deadly opportunistic human pathogen with high mortality worldwide. Notably, climate warming is likely to expand its geographical range and increase the infection risk for individuals in coastal regions. However, due to the absence of comprehensive surveillance systems, the emergence and characteristics of clinical V. vulnificus isolates remain poorly understood in China.

Methods

In this study, we investigate antibiotic resistance, virulence including serum resistance, and hemolytic ability, as well as molecular characteristics of 21 V. vulnificus isolates collected from patients in Ningbo, China.

Results and discussion

The results indicate that all isolates have been identified as potential virulent vcg C type, with the majority (16 of 21) classified as 16S rRNA B type. Furthermore, these isolates exhibit a high level of antibiotic resistance, with 66.7% resistance to more than three antibiotics and 61.9% possessing a multiple antibiotic resistance (MAR) index exceeding 0.2. In terms of virulence, most isolates were categorized as grade 1 in serum resistance, with one strain, S12, demonstrating intermediate sensitivity in serum resistance, belonging to grade 3. Whole genome analysis disclosed the profiles of antibiotic resistance genes (ARGs) and virulence factors (VFs) in these strains. The strains share substantial VF genes associated with adherence, iron uptake, antiphagocytosis, toxin, and motility. In particular, key VFs such as capsule (CPS), lipopolysaccharide (LPS), and multifunctional autoprocessing repeats-in-toxin (MARTX) are prevalent in all isolates. Specifically, S12 possesses a notably high number of VF genes (672), which potentially explains its higher virulence. Additionally, these strains shared six ARGs, namely, PBP3, adeF, varG, parE, and CRP, which likely determine their antibiotic resistance phenotype.

Conclusion

Overall, our study provides valuable baseline information for clinical tracking, prevention, control, and treatment of V. vulnificus infections.

Spatiotemporal Regulation of Vibrio Exotoxins by HlyU and Other Transcriptional Regulators

After invading a host, bacterial pathogens secrete diverse protein toxins to disrupt host defense systems. To ensure successful infection, however, pathogens must precisely regulate the expression of those exotoxins because uncontrolled toxin production squanders energy. Furthermore, inappropriate toxin secretion can trigger host immune responses that are detrimental to the invading pathogens. Therefore, bacterial pathogens use diverse transcriptional regulators to accurately regulate multiple exotoxin genes based on spatiotemporal conditions. This review covers three major exotoxins in pathogenic Vibrio species and their transcriptional regulation systems. When Vibrio encounters a host, genes encoding cytolysin/hemolysin, multifunctional-autoprocessing repeats-in-toxin (MARTX) toxin, and secreted phospholipases are coordinately regulated by the transcriptional regulator HlyU. At the same time, however, they are distinctly controlled by a variety of other transcriptional regulators. How this coordinated but distinct regulation of exotoxins makes Vibrio species successful pathogens? In addition, anti-virulence strategies that target the coordinating master regulator HlyU and related future research directions are discussed.

Phylogeny and life cycle of the zoonotic pathogen Vibrio vulnificus

Vibrio vulnificus is a zoonotic pathogen able to cause diseases in humans and fish that occasionally result in sepsis and death. Most reviews about this pathogen (including those related to its ecology) are clearly biased towards its role as a human pathogen, emphasizing its relationship with oysters as its main reservoir, the role of the known virulence factors as well as the clinic and the epidemiology of the human disease. This review tries to give to the reader a wider vision of the biology of this pathogen covering aspects related to its phylogeny and evolution and filling the gaps in our understanding of the general strategies that V. vulnificus uses to survive outside and inside its two main hosts, the human and the eel, and how its response to specific environmental parameters determines its survival, its death, or the triggering of an infectious process.

Exploring the Pathogenic Potential of Vibrio vulnificus Isolated from Seafood Harvested along the Mangaluru Coast, India

It has been observed that not all strains of Vibrio vulnificus are virulent. Determining the virulence of strains that are frequently present in seafood is of significance for ensuring seafood safety. This study is an attempt to predict the virulence of seafood-borne V. vulnificus isolated along the Mangaluru Coast, India. The isolates tested possessed a vcgC gene sequence with high similarity to that in the clinical strain. Transcriptional analysis of core virulence genes in seafood isolate E4010 showed the phenomenon of contact-mediated expression of rtxA1 which correlated well with the actin disintegration and cytotoxicity. These results suggest that the seafood isolates tested in this study possess a functional RtxA1 which could help in initiating the infection. However, other putative virulence genes such as vvpE encoding an extracellular protease, vvhA encoding hemolysin, flp encoding tad pilin and ompU encoding fibronectin-binding protein were also constitutively expressed. Virulence-associated attributes such as cytotoxicity and adherence matched the response of the clinical strain (p > 0.05). On the other hand, the environmental strains showed higher serum sensitivity compared with the clinical strain. These findings show that the part of virulence attributes required for the disease process might be intact in these isolates.

The transcriptome analysis of the Arabidopsis thaliana in response to the Vibrio vulnificus by RNA-sequencing

Because of the recent increase in the demand for fresh produce, contamination of raw food products has become an issue. Foodborne diseases are frequently caused by the infection of leguminous plants by human bacterial pathogens. Moreover, contamination by Vibrio cholerae, closely related with Vibrio vulnificus, has been reported in plants and vegetables. Here, we investigated the possibility of Vibrio vulnificus 96-11-17M, an opportunistic human pathogen, to infect and colonize Arabidopsis thaliana plants, resulting in typical disease symptoms at 5 and 7 days post-inoculation in vitro and in planta under artificial and favorable conditions, respectively. RNA-Seq analysis revealed 5,360, 4,204, 4,916 and 3,741 differentially expressed genes (DEGs) at 12, 24, 48 and 72 h post-inoculation, respectively, compared with the 0 h time point. Gene Ontology analysis revealed that these DEGs act in pathways responsive to chemical and hormone stimuli and plant defense. The expression of genes involved in salicylic acid (SA)-, jasmonic acid (JA)- and ethylene (ET)-dependent pathways was altered following V. vulnificus inoculation. Genetic analyses of Arabidopsis mutant lines verified that common pathogen-associated molecular pattern (PAMP) receptors perceive the V. vulnificus infection, thus activating JA and ET signaling pathways. Our data indicate that the human bacterial pathogen V. vulnificus 96-11-17M modulates defense-related genes and host defense machinery in Arabidopsis thaliana under favorable conditions.

Characterization of temperature-dependent hemin uptake receptors HupA and HvtA in Vibrio vulnificus

The Gram-negative pathogen Vibrio vulnificus produces several iron-sequestration systems including a hemin uptake system in response to iron limitation as a means to acquire this essential element. Strains of this organism are capable of causing serious septicemia in humans and eels, where hemin is abundant and an advantageous source of iron. Vibrio vulnificus hemin uptake systems consist of HupA, a well studied outer membrane protein, and a recently identified HvtA protein receptor. In this study, we confirmed that the expression of the hvtA gene is iron-regulated in a fur-dependent manner. When analyzed for virulence in a hemin-overloaded murine model system, the hupA gene was more important for establishing infection than the hvtA gene. Transcriptional profiling of these genes using strains of two different biotypes, biotype 1 (human pathogen) and biotype 2 (eel pathogen), showed that the expression of the two receptors was also regulated in response to temperature. The expression of hupA was highly induced in elevated temperatures in the human pathogenic strain when tested in iron-depleted conditions. Conversely, hvtA expression was induced significantly in the eel pathogenic strain at a lower temperature, a condition where the hupA locus was relatively repressed. Our results indicate that although both hupA and hvtA are involved for optimal hemin uptake in V. vulnificus, their expression is dually regulated by the environmental cues of iron concentration and temperature. Together, these data suggest that the virulence genes hupA and hvtA are tightly regulated and strictly induced during iron limitation combined with the physiological temperature of the host organism.

Potentially human-virulent Vibrio vulnificus isolates from diseased great pompano (Trachinotus goodei)

Vibrio vulnificus is an opportunistic human pathogen responsible for the majority of seafood-associated deaths worldwide and is also a relevant fish pathogen for the aquaculture industry. In addition to infections in aquatic livestock, V. vulnificus also represents a risk to aquarium animals. For the first time, this work describes an important mortality outbreak in Trachinotus goodei in a zoo aquarium, with the isolation of Vibrio vulnificus (Vv) from the internal organs of the diseased fish. The isolates were identified by MALDI-TOF MS, serotyped and characterized by pulsed-field gel electrophoresis (PFGE). Although the isolates from great pompanos did not belong to pathovar piscis (formerly biotype 2) or to any of the fish-related serovars, they all had identical phenotypes, antimicrobial susceptibility profiles and PFGE patterns, which together with their isolation in pure culture from internal organs is strongly indicative of their clinical significance. Moreover, Vv isolates harboured important genetic markers of human virulence potential: they had the clinical variant of the vcg gene, gave the 338 bp DNA amplification product of the pilF gene and resisted the bactericidal activity of human serum. All these results strongly suggest that these Vv isolates should be considered potentially virulent for humans. These results extend the range of fish species affected by V. vulnificus, confirm the threat that this pathogen represents to aquatic animals and highlight the risk that this bacterial pathogen poses to human health.

Comparative performance of TCBS and TSA for the enumeration of trh+ Vibrio parahaemolyticus by direct colony hybridization

Vibrio parahaemolyticus is one of the important foodborne pathogens is of public health concern due to the emergence of pandemic strains causing disease outbreaks worldwide. We evaluated the DNA based colony hybridization technique for the detection and enumeration of total and pathogenic V. parahaemolyticus from the bivalve shellfish, clam using non-radioactive, enzyme-labeled probe targeting the tlh and trh genes, respectively. The digoxigenin (DIG) labeled probes designed in this study showed 100% specificity by dot blot assay. Colony hybridization using DIG probes was performed using both non-selective, trypticase soy agar (TSA) and the selective medium, thiosulfate citrate bile salts sucrose (TCBS) agar. Of 32 clam samples analyzed, 71.88% had>10,000 V. parahaemolyticus cells/g in TSA whereas it was 18.75% in case of TCBS. All the samples showed the presence of total V. parahaemolyticus in TSA and 97% in the case of TCBS. Interestingly, results of the trh⁺V. parahaemolyticus samples were quite high while using TCBS plates (62.5%) as compared to TSA (43.75%). However, the cell numbers obtained from TSA were higher than from TCBS. Several yellow colonies on TCBS turned out to be V. parahaemolyticus using colony hybridization, which was further confirmed by PCR and sucrose utilization test. Colony hybridization using DIG-labeled probe was found to be highly sensitive and could differentiate and enumerate pathogenic and non-pathogenic strains of V. parahaemolyticus. Since traditional methods are not only labor-intensive and time-consuming but also less sensitive, colony hybridization using DIG-labeled probes would be a useful alternative for the enumeration of V. parahaemolyticus in naturally contaminated seafood.

A Mutation in the Mesorhizobium loti oatB Gene Alters the Physicochemical Properties of the Bacterial Cell Wall and Reduces Survival inside Acanthamoeba castellanii

In our previous report, we had shown that the free-living amoeba Acanthamoeba castellanii influenced the abundance, competiveness, and virulence of Mesorhizobium loti NZP2213, the microsymbiont of agriculturally important plants of the genus Lotus. The molecular basis of this phenomenon; however, had not been explored. In the present study, we demonstrated that oatB, the O-acetyltransferase encoding gene located in the lipopolysaccharide (LPS) synthesis cluster of M. loti, was responsible for maintaining the protective capacity of the bacterial cell envelope, necessary for the bacteria to fight environmental stress and survive inside amoeba cells. Using co-culture assays combined with fluorescence and electron microscopy, we showed that an oatB mutant, unlike the parental strain, was efficiently destroyed after rapid internalization by amoebae. Sensitivity and permeability studies of the oatB mutant, together with topography and nanomechanical investigations with the use of atomic force microscopy (AFM), indicated that the incomplete substitution of lipid A-core moieties with O-polysaccharide (O-PS) residues rendered the mutant more sensitive to hydrophobic compounds. Likewise, the truncated LPS moieties, rather than the lack of O-acetyl groups, made the oatB mutant susceptible to the bactericidal mechanisms (nitrosative stress and the action of lytic enzymes) of A. castellanii.

Occurrence of clinical genotype Vibrio vulnificus in clam samples in Mangalore, Southwest coast of India

Vibrio vulnificus is an opportunistic human pathogen causing gastroenteritis, wound infection and primary septicemia. V. vulnificus population has been divided into subpopulations based on their phenotype and genotype characteristics. In this study, 38.5% (10/26) of clam (Meretrix meretrix) samples obtained from Mangalore markets were seen to harbor V. vulnificus. Biochemical characterization of V. vulnificus isolates showed the strains to belong to Biotype 1 phenotype. Genotyping of strains using the 16S rRNA and virulence correlated gene (vcg) typing methods identified the isolates to be of 16S rRNA typeB and vcgC type respectively. Analysis of representative 16S rRNA and vcg gene sequences further substantiated that the V. vulnificus associated with clams in the present study to be of clinical origin, implicated as virulent type responsible for causing infection in humans.

The Biology of Vibrio vulnificus

Vibrio vulnificus, carrying a 50% fatality rate, is the most deadly of the foodborne pathogens. It occurs in estuarine and coastal waters and it is found in especially high numbers in oysters and other molluscan shellfish. The biology of V. vulnificus, including its ecology, pathogenesis, and molecular genetics, has been described in numerous reviews. This article provides a brief summary of some of the key aspects of this important human pathogen, including information on biotypes and genotypes, virulence factors, risk factor requirements and the role of iron in disease, association with oysters, geographic distribution, importance of salinity and water temperature, increasing incidence associated with global warming. This article includes some of our findings as presented at the "Vibrios in the Environment 2010" conference held in Biloxi, MS.

Fatores de patogenicidade de Vibrio spp. de importância em doenças transmitidas por alimentos

RESUMO: As bactérias do gênero Vibrio habitam ambiente tipicamente marinho e estuarino, sendo comumente isoladas de pescados. As principais espécies de Vibrio reportadas como agentes de infecções em humanos são V. vulnificus , V. parahaemolyticus , V. cholerae e V. mimicus . V. vulnificus é considerado o mais perigoso, podendo causar septicemia e levar à morte. V. parahaemolyticus é um patógeno importante nas regiões costeiras de clima temperado e tropical em todo o mundo e tem sido responsável por casos de gastroenterites associadas ao consumo de peixes, moluscos e crustáceos marinhos. V. cholerae causa surtos, epidemias e pandemias relacionados com ambientes estuarinos. V. mimicus pode causar episódios esporádicos de gastroenterite aguda e infecções de ouvido. A patogenicidade das bactérias está ligada à habilidade do micro-organismo em iniciar uma doença (incluindo entrada, colonização e multiplicação no corpo humano). Para que isso ocorra, os micro-organismos fazem uso de diversos fatores. O objetivo desta revisão foi sintetizar o conhecimento disponível na literatura sobre os fatores de patogenicidade de V. vulnificus , V. parahaemolyticus , V. cholerae e V. mimicus .

The Fish Pathogen Vibrio vulnificus Biotype 2: Epidemiology, Phylogeny, and Virulence Factors Involved in Warm-Water Vibriosis

Vibrio vulnificus biotype 2 is the etiological agent of warm-water vibriosis, a disease that affects eels and other teleosts, especially in fish farms. Biotype 2 is polyphyletic and probably emerged from aquatic bacteria by acquisition of a transferable virulence plasmid that encodes resistance to innate immunity of eels and other teleosts. Interestingly, biotype 2 comprises a zoonotic clonal complex designated as serovar E that has extended worldwide. One of the most interesting virulence factors produced by serovar E is RtxA1 3 , a multifunctional protein that acts as a lethal factor for fish, an invasion factor for mice, and a survival factor outside the host. Two practically identical copies of rtxA1 3 are present in all biotype 2 strains regardless of the serovar, one in the virulence plasmid and the other in chromosome II. The plasmid also contains other genes involved in survival and growth in eel blood: vep07 , a gene for an outer membrane (OM) lipoprotein involved in resistance to eel serum and vep20 , a gene for an OM receptor specific for eel-transferrin and, probably, other related fish transferrins. All the three genes are highly conserved within biotype 2, which suggests that they are under a strong selective pressure. Interestingly, the three genes are related with transferable plasmids, which emphasizes the role of horizontal gene transfer in the evolution of V. vulnificus in nutrient-enriched aquatic environments, such as fish farms.

Draft Genome Sequence of Fish Pathogenic Vibrio vulnificus Biotype 2

Vibrio vulnificus is a marine pathogen capable of causing severe soft tissue infections and septicemia in humans. V. vulnificus biotype 2 is the etiological agent of fish vibriosis. We describe here the first draft genome sequence of V. vulnificus biotype 2, strain ES-7601, isolated from an infected eel in Japan.

Vibrio vulnificus outbreaks in Dutch eel farms since 1996: strain diversity and impact

Vibrio vulnificus is a potentially zoonotic bacterial pathogen of fish, which can infect humans (causing necrotic fasciitis). We analysed 24 V. vulnificus isolates (from 23 severe eel disease outbreaks in 8 Dutch eel farms during 1996 to 2009, and 1 clinical strain from an eel farmer) for genetic correlation and zoonotic potential. Strains were typed using biotyping and molecular typing by high-throughput multilocus sequence typing (hiMLST) and REP-PCR (Diversilab®). We identified 19 strains of biotype 1 and 5 of biotype 2 (4 from eels, 1 from the eel farmer), that were subdivided into 8 MLST types (ST) according to the international standard method. This is the first report of V. vulnificus biotype 1 outbreaks in Dutch eel farms. Seven of the 8 STs, of unknown zoonotic potential, were newly identified and were deposited in the MLST database. The REP-PCR and the MLST were highly concordant, indicating that the REP-PCR is a useful alternative for MLST. The strains isolated from the farmer and his eels were ST 112, a known potential zoonotic strain. Antimicrobial resistance to cefoxitin was found in most of the V. vulnificus strains, and an increasing resistance to quinolones, trimethoprim + sulphonamide and tetracycline was found over time in strain ST 140. Virulence testing of isolates from diseased eels is recommended, and medical practitioners should be informed about the potential risk of zoonotic infections by V. vulnificus from eels for the prevention of infection especially among high-risk individuals. Additional use of molecular typing methods such as hiMLST and Diversilab® is recommended for epidemiological purposes during V. vulnificus outbreaks.

Host-nonspecific iron acquisition systems and virulence in the zoonotic serovar of Vibrio vulnificus

The zoonotic serovar of Vibrio vulnificus (known as biotype 2 serovar E) is the etiological agent of human and fish vibriosis. The aim of the present work was to discover the role of the vulnibactin- and hemin-dependent iron acquisition systems in the pathogenicity of this zoonotic serovar under the hypothesis that both are host-nonspecific virulence factors. To this end, we selected three genes for three outer membrane receptors (vuuA, a receptor for ferric vulnibactin, and hupA and hutR, two hemin receptors), obtained single and multiple mutants as well as complemented strains, and tested them in a series of in vitro and in vivo assays, using eels and mice as animal models. The overall results confirm that hupA and vuuA, but not hutR, are host-nonspecific virulence genes and suggest that a third undescribed host-specific plasmid-encoded system could also be used by the zoonotic serovar in fish. hupA and vuuA were expressed in the internal organs of the animals in the first 24 h of infection, suggesting that they may be needed to achieve the population size required to trigger fatal septicemia. vuuA and hupA were sequenced in strains representative of the genetic diversity of this species, and their phylogenies were reconstructed by multilocus sequence analysis of selected housekeeping and virulence genes as a reference. Given the overall results, we suggest that both genes might form part of the core genes essential not only for disease development but also for the survival of this species in its natural reservoir, the aquatic environment.

Role of capsular polysaccharide (CPS) in biofilm formation and regulation of CPS production by quorum-sensing in Vibrio vulnificus

Extracellular polysaccharides, such as lipopolysaccharide and loosely associated exopolysaccharides, are essential for Vibrio vulnificus to form biofilms. The role of another major component of the V. vulnificus extracellular matrix, capsular polysaccharide (CPS), which contributes to colony opacity, has been characterized in biofilm formation. A CPS-deficient mutant, whose wbpP gene encoding UDP-GlcNAc C4-epimerase was knocked out, formed significantly more biofilm than wild type, due to increased hydrophobicity of the cell surface, adherence to abiotic surfaces and cell aggregation. To elucidate the direct effect of CPS on biofilm structure, extracted CPS and a CPS-degrading enzyme, α-N-acetylgalactosaminidase, were added in biofilm assays, resulting in reduction and increment of biofilm sizes respectively. Therefore, it is suggested that CPS play a critical role in determining biofilm size by restricting continual growth of mature biofilms. Since CPS is required after maturation, CPS biosynthesis should be controlled in a cell density-dependent manner, e.g. by quorum-sensing (QS) regulation. Analysing transcription of the CPS gene cluster revealed that it was activated by SmcR, a QS master regulator, via binding to the upstream region of the cluster. Therefore, CPS was produced when biofilm cell density reached high enough to turn on QS regulation and limited biofilms to appropriate sizes.

Genome sequence of the human-pathogenic bacterium Vibrio vulnificus type strain ATCC 27562

Vibrio vulnificus, which is, like Vibrio cholerae and Vibrio parahaemolyticus, a pathogen of humankind, is a Gram-negative, curved, motile, and rod-shaped bacterium [corrected] Here, we present the draft genome sequence of the type strain, ATCC 27562, which was the first isolated Vibrio vulnificus strain.

Intraspecific Differentiation of Vibrio vulnificus Biotypes by Amplified Fragment Length Polymorphism and Ribotyping

The intraspecific genomic relatedness of 80 Vibrio vulnificus isolates, 44 of biotype 1 and 36 of biotype 2, from different geographic origins and sources was evaluated by ribotyping and AFLP (amplified fragment length polymorphism) fingerprinting. Ribopatterns of DNAs digested with KpnI and hybridized with an oligonucleotide complementary to a highly conserved sequence in the 23S rRNA gene revealed up to 19 ribotypes in the species, which were different for the two biotypes. Sixteen different ribotypes were found within biotype 1 strains from clinical and environmental sources, and only three, recovered mainly from diseased eels, were found within biotype 2. Within this biotype, 96% of the strains showed the same ribopattern. The closest similarity was shown by the strains coming from the same eel farm, irrespectively of biotype. AFLP fingerprints obtained by selective PCR amplification of HindIII-TaqI double-restricted DNA fragments exhibited a strain-specific pattern which allowed the finest differentiation of subgroups within the eel-pathogenic isolates sharing the same ribopattern. Both techniques revealed good genetic markers for intraspecific differentiation of V. vulnificus. Ribotyping clearly separated the eel-pathogenic strains from the clinical and environmental isolates, whereas AFLP enabled the monitoring of individual strains and therefore constitutes one of the most discriminative tools for epidemiological and ecological studies.

Genetic characterization of Vibrio vulnificus strains from tilapia aquaculture in Bangladesh

Outbreaks of Vibrio vulnificus wound infections in Israel were previously attributed to tilapia aquaculture. In this study, V. vulnificus was frequently isolated from coastal but not freshwater aquaculture in Bangladesh. Phylogenetic analyses showed that strains from Bangladesh differed remarkably from isolates commonly recovered elsewhere from fish or oysters and were more closely related to strains of clinical origin.

A comparative epizootiologic study of the two fish-pathogenic serovars of Vibrio vulnificus biotype 2

Vibrio vulnificus biotype 2 is subdivided into two main serovars, serovar E, able to infect fish and humans, and serovar A, only virulent for fish. Serovar E emerged in 1976 as the causative agent of a haemorrhagic septicaemia (warm-water vibriosis) affecting eels cultured in brackish water. Serovar A emerged in 2000 in freshwater-cultured eels vaccinated against serovar E, causing warm-water vibriosis with fish showing a haemorrhagic intestine as the main differential sign. The aim of the present work was to compare the disease caused by both serovars in terms of transmission routes, portals of entry and host range. Results of bath, patch-contact and oral-anal challenges demonstrated that both serovars spread through water and infect healthy eels, serovar A entering mainly by the anus and serovar E by the gills. The course of the disease under laboratory conditions was similar for both serovars in terms of transmission and dependence of degree of virulence on water parameters (temperature and salinity). However, the decrease in degree of virulence in fresh water was significantly greater in serovar E than in serovar A. Finally, both serovars proved pathogenic for tilapia, sea bass and rainbow trout, but not for sea bream, with significant differences in degree of virulence only in rainbow trout. In conclusion, serovar A seems to represent a new antigenic form of V. vulnificus biotype 2 with an unusual portal of entry and is better adapted to fresh water than serovar E.

Vibrio vulnificus produces quorum sensing signals of the AHL-class

Vibrio vulnificus is an aquatic pathogenic bacterium that can cause vibriosis in humans and fish. The species is subdivided into three biotypes with the fish-virulent strains belonging to biotype 2. The quorum sensing (QS) phenomenon mediated by furanosyl borate diester or autoinducer 2 (AI-2) has been described in human strains of biotype 1, and here we show that the luxS gene which encodes AI-2 is present in all strains of V. vulnificus regardless of origin, biotype or serovar. In this study, we also demonstrate that V. vulnificus produces QS signals of the acylated homoserine lactone (AHL) class (AI-1). AHLs were detected in strains of biotype 1 and 2 from water, fish and human wound infections but not in strains isolated from human septicaemic cases. The AHL compound was identified as N-butanoyl-homoserine-lactone (C(4)-HL) by both reporter strains and by HPLC-high-resolution MS. C(4)-HL was detected when AHL-positive strains were grown in low-nutrient medium [modified sea water yeast extract (MSWYE)] but not in rich media (tryptic soy broth or brain-heart infusion) and its production was enhanced when blood factors were added to MSWYE. C(4)-HL was detected in vivo, in eels infected with AHL-positive biotype 2 strains. No known AHL-related gene was detected by PCR or Southern blot suggesting that AHL-related genes in V. vulnificus are different from those found in other Gram-negative bacteria.

Plasmid diversity in Vibrio vulnificus biotypes

Vibrio vulnificus is a heterogeneous bacterial species that can be virulent for humans and fish. Virulence in fish seems to rely on a recently described plasmid that can be transmitted between strains, aided by a conjugative plasmid. The main objective of this work was to analyse the plasmid content of a wide collection of strains from the three biotypes of the species, as well as to identify putative conjugative and virulence plasmids by means of Southern hybridization with specific probes and sequence analysis of selected gene markers. We found 28 different plasmid profiles in a total of 112 strains, which were relatively biotype- or serovar-specific. Biotype 1 lacked high-molecular-mass plasmids, with the exception of a putative conjugative plasmid of 48 kb that was present in 42.8% of clinical and environmental strains isolated worldwide. All biotype 2 strains possessed the virulence plasmid, whose molecular mass ranged between 68 and 70 kb, and 89.65% of these strains also had a putative conjugative plasmid with a molecular size of 52-56 kb. Finally, a 48 kb putative conjugative plasmid was present in all biotype 3 strains. Data from partial sequencing of traD, traI and the whole vep07 (a recently described plasmid-borne virulence gene) from a selection of strains suggest that the plasmids of 48-56 kb probably belong to the same family of F-plasmids as pYJ016 and that the gene vep07 is absolutely essential for fish virulence. Additional cryptic plasmids of low molecular mass were present in the three biotypes. In conclusion, plasmids are widespread among V. vulnificus species and could contribute substantially to genetic plasticity of the species.

A common virulence plasmid in biotype 2 Vibrio vulnificus and its dissemination aided by a conjugal plasmid

Strains of Vibrio vulnificus, a marine bacterial species pathogenic for humans and eels, are divided into three biotypes, and those virulent for eels are classified as biotype 2. All biotype 2 strains possess one or more plasmids, which have been shown to harbor the biotype 2-specific DNA sequences. In this study we determined the DNA sequences of three biotype 2 plasmids: pR99 (68.4 kbp) in strain CECT4999 and pC4602-1 (56.6 kb) and pC4602-2 (66.9 kb) in strain CECT4602. Plasmid pC4602-2 showed 92% sequence identity with pR99. Curing of pR99 from strain CECT4999 resulted in loss of resistance to eel serum and virulence for eels but had no effect on the virulence for mice, an animal model, and resistance to human serum. Plasmids pC4602-2 and pR99 could be transferred to the plasmid-cured strain by conjugation in the presence of pC4602-1, which was self-transmissible, and acquisition of pC4602-2 restored the virulence of the cured strain for eels. Therefore, both pR99 and pC4602-2 were virulence plasmids for eels but not mice. A gene in pR99, which encoded a novel protein and had an equivalent in pC4602-2, was further shown to be essential, but not sufficient, for the resistance to eel serum and virulence for eels. There was evidence showing that pC4602-2 may form a cointegrate with pC4602-1. An investigation of six other biotype 2 strains for the presence of various plasmid markers revealed that they all harbored the virulence plasmid and four of them possessed the conjugal plasmid in addition.

Phenotypic and genotypic characterization of a new fish-virulent Vibrio vulnificus serovar that lacks potential to infect humans

Vibrio vulnificus is a bacterial species that is virulent for humans and fish. Human isolates are classified into biotypes 1 and 3 (BT1 and BT3) and fish isolates into biotype 2 (BT2). However, a few human infections caused by BT2 isolates have been reported worldwide (zoonosis). These BT2 human isolates belong to serovar E (SerE), which is also present in diseased fish. The aim of the present work was to characterize a new BT2 serovar [serovar A (SerA)], which emerged in the European fish-farming industry in 2000, by means of phenotypic, serological and genetic [plasmid profiling, ribotyping and random amplified polymorphic DNA (RAPD)] methodologies. The results confirmed that SerA constitutes a homogeneous O-serogroup within the species that shares plasmidic information with SerE. Like SerE, this new serogroup was resistant to fresh fish serum, as well as being highly virulent for fish. In contrast, it was sensitive to human serum and avirulent for mice, even after pretreatment with iron. The two serovars presented different biochemical profiles as well as specific patterns by ribotyping and RAPD analysis. In conclusion, SerA seems to constitute a different clonal group that has recently emerged within the species V. vulnificus, with pathogenic potential for fish but not for humans.

Serological and molecular characteristics of Vibrio vulnificus biotype 3: evidence for high clonality

Vibrio vulnificus biotype 3 has been implicated as the causative pathogen of an ongoing disease outbreak that erupted in Israel in 1996. Recent work based on multi-locus sequence typing (MLST) showed that V. vulnificus biotype 3 is genetically homogeneous. The aim of this study was to investigate the existence of subpopulations within this homogeneous biotype by characterizing the surface antigens and analysing the sequence diversity of selected outer-membrane protein (OMP)-encoding genes. Rabbit antisera were prepared against biotype 1, 2 and 3 strains. The results of the slide-agglutination test, dot-blot assay (using fresh and boiled cells), and immunoblotting of lipopolysaccharides (LPS) and OMPs were evaluated. By slide-agglutination and dot-blot assays all biotype 3 strains agglutinated with the selected biotype 3 strain. This homogeneity was supported by immunoblot analysis of the LPS. Analysis of OMP patterns revealed that all three biotypes share a considerable number of common bands that are antigenically related. Cluster analysis of DNA sequence data from selected OMP-encoding genes showed that biotype 3 strains form a genetically distinct and homogeneous clone. The homogeneity of surface antigens and the lack of any sequence diversity among both housekeeping and OMP-encoding genes reaffirms the highly clonal nature of biotype 3 and suggests that it has only recently descended from the parent population of V. vulnificus.

Thermal factors influencing detection of Vibrio vulnificus using real-time PCR

Five thermal factors, including initial denaturation temperature, cycling denaturation temperature, annealing temperature, extension temperature and the temperature at which the intensity of the fluorescent signal is read, were evaluated for their effects on the detection of Vibrio vulnificus via real-time PCR. Fluorescent signal detection after extension was set between the Tm value of the primer-dimers (79 degrees C) and that of the PCR target amplicons (84 degrees C). This effectively eliminated the overestimation of the yield of PCR amplicons due to the presence of primer-dimers which otherwise led to erroneously lower Ct values (1.91+/-0.22 cycles lower). The annealing and extension steps were combined to convert a three-step PCR to a two-step PCR. This consisted of initial denaturation at 95 degrees C for 3 min, cycling denaturation at 94 degrees C for 15 s and a combined annealing and extension step at 60 degrees C for 5 s in each PCR cycle. One genomic target per real-time PCR reaction was detected with the simplified two-step PCR.

Rapid quantification of Vibrio vulnificus in clams (Protochaca staminea) using real-time PCR

We used a rapid DNA extraction and purification method to obtain the DNA from Vibrio vulnificus seeded into clam tissue homogenates for real-time PCR quantification of the organism. Without enrichment, the limit of detection was 1 x 10(2) cfu/g of tissue with a linear detection range of 1 x 10(2) to 1 x 10(8) cfu/g. With a 5 h non-selective enrichment, the limit of detection was 1 cfu/g of tissue with a linear detection range of 1 to 1 x 10(6) cfu/g of tissue. We found a 10-fold higher detection limit with seeded clam tissue homogenates compared to pure culture in TSB(+). The detection limits with pure broth culture and seeded tissue homogenates were identical, 1 cfu/ml and 1 cfu/ml, respectively, following 5 h non-selective enrichment. However, the Ct value with tissue homogenates was about 3 threshold cycles higher than with pure culture.

Role of NtrC in biofilm formation via controlling expression of the gene encoding an ADP-glycero-manno-heptose-6-epimerase in the pathogenic bacterium,Vibrio vulnificus

To identify the genetic elements required for biofilm formation, we screened a pool of random Vibrio vulnificus mutants for their ability to form biofilms. One mutant displaying significantly decreased biofilm-forming activity was found to contain a transposon insertion in the ntrC gene. The ntrC gene encodes a well-known transcriptional activator. We examined how this regulator modulates a biofilm-forming process in V. vulnificus by searching for NtrC target gene(s). Comparison of the proteomes of ntrC mutant and wild-type strains grown under planktonic and biofilm stages revealed that synthesis of the protein homologous to GmhD (ADP-glycero-manno-heptose-6-epimerase) was elevated during the growth period for biofilm formation and was strongly influenced by NtrC. A luxAB-transcriptional fusion with the gmhD promoter region indicated that gmhD expression was positively regulated by both NtrC and RpoN. The function of the gmhD gene product in V. vulnificus was assessed by constructing and phenotypic analyses of an isogenic mutant. The gmhD mutant was defective in production of mature lipopolysaccharide (LPS) and exopolysaccharides (EPS), and demonstrated an attenuated ability to form a biofilm. These results suggest that NtrC acts as a key regulator of both LPS and EPS biosyntheses and, thereby, modulates critical steps in biofilm development of V. vulnificus.

Vibrio vulnificus serovar A: an emerging pathogen in European anguilliculture

The spread of the emerging pathogen Vibrio vulnificus biotype 2 serovar A in Danish anguilliculture is reported. Serovar A was originally isolated in a Spanish eel farm in 2000 and occurred in Denmark in the summer of 2004, affecting eels of 5-10 g body weight cultured in fresh water. The Danish eels showed clinical signs different from those reported for Spanish eels, such as severe haemorrhages in the head and gill region with necrosis of the soft tissues. Danish isolates were biochemically and serologically identical to Spanish serovar A strains and also highly virulent for eels by both intraperitoneal injection and immersion challenges. Vaccination with Vulnivaccine, a vaccine against V. vulnificus serovar E, cross-protected eels against serovar A. The LD(50) for experimentally infected vaccinated animals was significantly higher than for non-vaccinated animals.

Detection and differentiation of Vibrio vulnificus in seawater and plankton of a coastal zone of the Mediterranean Sea

Vibrio vulnificus, a human and animal pathogen, is present in low numbers in the Mediterranean Sea. Seawater and plankton samples were collected from a marine coastal zone of the Straits of Messina in the Mediterranean Sea (Italy) in order to investigate V. vulnificus as free-living (>0.2 microm) and associated with small (>64 microm) and large plankton (>200 microm) utilizing cultural and molecular techniques. Characteristic colonies, grown on thiosulfate, citrate, bile salts and sucrose agar plates, were identified using a biochemical protocol system. A PCR assay was used to confirm isolates and to directly detect V. vulnificus in environmental concentrated samples. Specific primers were used to target the structural cytotoxin/hemolysin gene and the variable regions of 16S rRNA species-specific for V. vulnificus. In addition, a tri-primer PCR of 16S rRNA was used for the differentiation of V. vulnificus strains. Direct detection in marine samples was more frequent than isolation of culturable forms. All isolates were assigned to V. vulnificus biotype 1, 16S rRNA type B. These results confirm the low incidence of V. vulnificus in Mediterranean coastal waters. The isolation of cultivable forms is limited to the warmest months. 16S rRNA primers were the most sensitive molecular tool as they allowed detection of V. vulnificus in 79.1% of samples. Due to the low incidence of V. vulnificus in the Mediterranean coastal environment, its detection requires a molecular approach. The occurrence of V. vulnificus as plankton-associated confirms the role of plankton as a potential reservoir for this pathogen.

Identification of DNA sequences specific for Vibrio vulnificus biotype 2 strains by suppression subtractive hybridization

Vibrio vulnificus can be divided into three biotypes, and only biotype 2, which is further divided into serovars, contains eel-virulent strains. We compared the genomic DNA of a biotype 2 serovar E isolate (tester) with the genomic DNAs of three biotype 1 strains by suppression subtractive hybridization and then tested the distribution of the tester-specific DNA sequences in a wide collection of bacterial strains. In this way we identified three plasmid-borne DNA sequences that were specific for biotype 2 strains irrespective of the serovar and three chromosomal DNA sequences that were specific for serovar E biotype 2 strains. These sequences have potential for use in the diagnosis of eel vibriosis caused by V. vulnificus and in the detection of biotype 2 serovar E strains.

Monitoring of active but non-culturable bacterial cells by flow cytometry

Flow cytometric signatures (i.e., light scatter, red and green fluorescence) were obtained for the active but non-culturable (ABNC) cells of E. coli and a coliform isolate H03N1, in seawater microcosms using BacLight, a live-dead assay kit from Molecular Probes (Eugene/Portland, OR). Previous studies have reported that there are two major adaptations, which cells undergo during the formation of ABNC states: cell wall toughening and DNA condensation. Therefore, we hypothesized that the matured ABNC forms should be more resistant to extreme temperature treatments (i.e., by freezing in liquid nitrogen and thawing at room temperature) than the normal and transition populations. It was shown that the membrane-compromised cells (comprising of normal wild-type and dead cells which are less resistant to rapid freeze thaw) could be differentiated from the matured ABNC using BacLight staining and fluorescence detection by flow cytometry. The population of ABNC cells, which could not be cultured using m-FC media (for the enumeration of fecal coliforms), was resuscitated in phosphate buffer saline followed by growth in Luria broth. Flow cytometry was thus able to detect and differentiate the ABNC cells against a mixed population comprising of culturable cells, transition populations, and dead cells. The results also showed that the formation of ABNC is as early as 2 days in seawater microcosms. By directly comparing the coliform levels enumerated by the BacLight based flow cytometry assays and m-FC technique, it was shown that the presence of coliforms can be undetected by the membrane filtration method.

Protocol for specific isolation of virulent strains of Vibrio vulnificus serovar E (biotype 2) from environmental samples

The eel pathogen Vibrio vulnificus biotype 2 comprises at least three serovars, with serovar E being the only one involved in both epizootics of eel vibriosis and sporadic cases of human infections. The virulent strains of this serovar (VSE) have only been recovered from clinical (mainly eel tissue) sources. The main objective of this work was to design and validate a new protocol for VSE-specific isolation from environmental samples. The key element of the new protocol is the broth used for the first step (saline eel serum broth [SEB]), which contains eel serum as a nutritive and selective component. This approach takes advantage of the ability of VSE cells to grow in eel serum and thus to separate themselves from the pool of competitors. The growth yield in SEB after 8 h of incubation was 1,000 times higher for VSE strains than for their putative competitors (including biotype 1 strains of the species). The selective and differential agar Vibrio vulnificus medium (VVM) was selected from five selective media for the second step because it gave the highest plating efficiency not only for the VSE group but also for other V. vulnificus groups, including biotype 3. The entire protocol was validated by field studies, with alkaline peptone water plus VVM as a control. V. vulnificus was isolated by both protocols, but serovar E was only recovered by the new method described here. All selected serovar E isolates were identified as VSE since they were virulent for both eels and iron-overloaded mice and resisted the bactericidal action of eel and iron-overloaded human sera. In conclusion, this new protocol is a suitable method for the isolation of VSE strains from environmental samples and is recommended for epidemiological studies of the pathogenic serovar E.

Occurrence of Vibrio vulnificus in fish and shellfish available from markets in China

Vibrio vulnificus is a naturally occurring estuarine bacterium often associated with disease such as septicemia in humans following consumption of raw and lightly cooked seafood. In China and neighboring countries, rapid economic growth has encouraged increased consumption of seafood, and dietary habits are changing, with more people eating raw fish. In this study, the prevalence of V. vulnificus was investigated in 48 samples from 11 species of live seafood available from markets in coastal cities of China. The bacterium was detected in four of four razor clam samples, in seven of seven giant tiger prawn samples, and in five of nine mantis shrimp samples. The bacterium was also found in water samples of the prawn aquaria at the markets. The maximum level of V. vulnificus was 3.4 log CFU/g in the razor clam samples and 4.9 log CFU/g in the prawn samples by a direct spreading method. Differential bacterial counts on the prawn body revealed that most of the bacteria were found on the shells (exoskeletons), with very few in the edible muscle. However, dense populations can be found in the intestines. Biochemical tests indicated that the isolates of V. vulnificus were biotype 1 strain, which is pathogenic to humans. These isolates were susceptible to ampicillin, penicillin, kanamycin, streptomycin, and erythromycin. These results suggest that V. vulnificus is a potential health hazard to humans in cities consuming and handling live shellfish, especially giant tiger prawns.

Influence of aquatic microbiota on the survival in water of the human and eel pathogen Vibrio vulnificus serovar E

The eel and human pathogen Vibrio vulnificus serovar E (biotype 2) is seldom isolated from natural waters, although it can survive in sterilized artificial seawater microcosms for years. The main objective of the present study was to investigate whether aquatic microbiota can limit its survival and recovery from water samples. A set of preliminary experiments of survival in microcosms containing natural seawater and water from eel farms showed that the persistence of this pathogen was mainly controlled by grazing, and secondarily by bacterial competition. The bacterial competition was further analysed in artificial seawater microcosms co-inoculated with selected virulent serovar E (VSE) strains and potential competitors. Competitors included V. vulnificus biotype 1 isolates and strains of selected species that can grow on the selective media designed for V. vulnificus isolation from water samples. Evidences of bacterial competition that was detrimental for VSE recovery were recorded. Thus, some species produced a deleterious effect on VSE strains under starvation, and others were able to use the resources more efficiently under nutrient input. These results suggest that an overgrowth of more efficient competitor bacteria in conventional media used for isolation of V. vulnificus could mask the recovery of VSE strains and explain the scarcity of reports on the isolation of this human and eel pathogen from natural waters.

Sequence polymorphism of the 16S rRNA gene of Vibrio vulnificus is a possible indicator of strain virulence

Vibrio vulnificus exhibits considerable strain-to-strain variation in virulence. Attempts to associate phenotypic or genotypic characteristics with strain virulence have been largely unsuccessful. Based on a 17-nucleotide difference throughout the sequence of the small subunit 16S rRNA gene, there are two major groups of V. vulnificus designated types A and B. In a survey of the 16S rRNA genotype in 67 V. vulnificus human clinical and nonclinical strains, we determined that the majority of nonclinical isolates are type A (31 of 33) and that there is a statistically significant association between the type B genotype and human clinical strains (26 of 34).

A rapid and improved method for the detection of Vibrio parahaemolyticus and Vibrio vulnificus strains grown on hydrophobic grid membrane filters

DNA probe-based detection methods were developed and characterized as an alternative to time-consuming and less specific conventional protocols. Digoxigenin-labeled probes were prepared by polymerase chain reaction amplification of the targeted sequences in the specific amplicons generated from genomic DNA. Specific probes with high yields were generated for the detection of the tlh gene of Vibrio parahaemolyticus and the cth gene of V. vulnificus. Colony (Southern) hybridization analyses were carried out using hydrophobic grid membrane filters (HGMFs) to allow biotype-specific differentiation of the two species. Eight strains of V. vulnificus and five strains of V. parahaemolyticus, including one standard (ATCC) strain of each biotype, were examined. Colony lysis, hybridization, and nonradioactive detection parameters were optimized for identification of the target biotypes arranged on the same HGMF and also on a conventional nylon membrane, thereby confirming the specificity of the probes and the comparative usefulness of the HGMFs. The experimental procedure presented here can be completed in 1 day. The protocol was designed specifically to identify the target Vibrio spp. and could potentially be used for the enumeration and differentiation of V. parahaemolyticus and V. vulnificus in foods.